This is old news. Myamoto said that he might add a multiplayer feature in the game, whether it will co-op, races or time attacks is still unknown. He also mentioned that you will have to earn it through meeting certain requirements within the game...if he does add it. More than likely it's going to happen since he heavily speaks about Luigi making an appearance.

Ever wondered about the term 1-4 player multiplayer, surely it should be 2-4 player multiplayer ^_^ if it were just the 1 person it would be single player. Silly game people, also I'm not sure how it could work...co op missions perhaps? That would be quite cool though would have to be extras. Or some kind of battle mode.